Video cassette recorders (referred to hereinafter as VCRs) etc. are conventionally provided with a picture quality correcting device for correcting image picture quality. There are several configurations possible for picture quality correction depending on the nature of the correction (such as noise suppression, correction of the frequency characteristic, or contour correction). Here, as a representative case, an explanation follows of a reproduction system provided in a VCR in which a luminance signal processing section carries out noise suppression and corrects the frequency characteristic.
As shown in FIG. 15, an FM signal (a frequency modulated luminance signal) from a video head, not shown, is demodulated into a luminance signal by an FM demodulator 61 in the luminance signal processing section, and is then sent to a noise canceller 62. In the noise canceller 62, a noise component is extracted from the luminance signal by a high-pass filter, not shown; then, this noise component is removed from the original luminance signal.
In a YNR (line correlation noise reduction circuit) 63 provided subsequent to the noise canceller 62, a noise component which is not in line correlation is excluded from the luminance signal. This is done by processing the luminance signal and another signal derived from it which is delayed by 1 H, utilizing the fact that the luminance signals corresponding to two adjacent lines are in correlation.
Further, in a picture-tone circuit 64 provided subsequent to the YNR 63, a high-frequency component of the luminance signal is corrected, the amount of correction corresponding to a control voltage generated by a picture quality adjuster VR.sub.61. The control voltage is adjusted by manipulating the picture quality adjuster VR.sub.61.
As shown in FIG. 16 by a broken line and an alternate long and short dash line, the picture-tone circuit 64 can increase or decrease a gain of 1 MHz or more by a maximum of approximately 10 dB. Furthermore, as shown in FIG. 17 by a broken line and an alternate long and short dash line, the picture-tone circuit 64 can increase or decrease a group delay of 1 MHz or more by a maximum of approximately 200 nsec. Using this picture-tone circuit 64 allows variation of the picture quality of the reproduced image to become linear with respect to the control voltage, as shown in FIG. 18.
Furthermore, the noise canceller 62 is arranged to switch the amount of noise suppression by changing the amplitude of the high-frequency component drawn out from the luminance signal. This amplitude of the high-frequency component changes according to the change in the limiting level of a limiter, not shown, provided in the noise canceller 62. This limiting level is set by resistors R.sub.61 and R.sub.62. In an SP (Standard Play) mode, the limiting level is set by the resistor R.sub.61, and in an LP (Long Play) mode, the limiting level is switched by switching a transistor Tr.sub.61 to ON so that the resistors R.sub.62 and R.sub.61 are connected in parallel.
On the other hand, when a recursive comb filter, for example, serves as the YNR 63, the YNR 63 is arranged to switch the amount of noise suppression by changing the amount of feedback in the input end of a 1 H delay element provided in the recursive comb filter, the amount of feedback being set by resistors R.sub.63 and R.sub.64. In the SP mode, the amount of feedback is set by the resistor R.sub.63 and in the LP mode the amount of feedback is changed by switching a transistor Tr.sub.62 to ON so that the resistors R.sub.64 and R.sub.63 are connected in parallel.
The switching ON and OFF of the transistors Tr.sub.61 and Tr.sub.62 is controlled by a control signal supplied to the respective bases via resistors R.sub.65 and R.sub.66. The control signal is released by a microcomputer 65 used in VCRs for system control, and is low level in the SP mode and high level in the LP mode.
With the arrangement described above, while the noise canceller 62 decreases noise in the horizontal direction of the image by eliminating the noise component in one line of the luminance signal, the YNR 63 decreases noise in the vertical direction of the image by using two adjacent lines to eliminate the noise component. The picture-tone circuit 64 makes it possible to improve the apparent definition of the image by emphasizing the high-frequency component of a luminance signal which renders detailed portions of the image.
When information recorded in a master video tape is copied (referred to hereinafter as dubbing) onto another video tape using a VCR provided with the arrangement described above, although the picture quality of the dubbed image has already been corrected by the play-back VCR, on playing back the dubbed image, picture quality correction is carried out again. That is, picture quality correction ends up being carried out twice. Consequently, when dubbing is performed, a somewhat diffused image is the result, although there is a substantial suppression of noise.
In order to surmount this problem and to retain detailed portions of the image as far as possible, recently developed home-VCRs have been provided with a function known as edit mode. In the edit mode function, picture quality correction is limited to that which takes place during dubbing. An explanation follows hereinbelow of a picture quality correcting device of a VCR provided with this function.
As shown in FIG. 19, the picture quality correcting device comprises a noise canceller 71, a YNR (line correlation noise reduction circuit) 72, and a picture-tone circuit 73. The output end of an edit switch 74 is connected to the noise canceller 71 and the YNR 72. The picture-tone circuit 73 is set to be connected to the output terminal of a picture quality adjuster VR.sub.71 when the edit switch 74 is OFF, and to a DC source 76 when the edit switch 74 is ON. This is done by means of a change-over switch 75.
In the picture quality correcting device arranged as described above, when the edit mode is selected by switching the edit switch 74 to ON, a voltage V.sub.ED is applied to the noise canceller 71 and the YNR 72. When this happens, the noise canceller 71 decreases the amount of noise suppression and the YNR 72 either decreases the amount of noise suppression or stops the suppression thereof.
The picture-tone circuit 73, on the other hand, flattens the corrected frequency characteristic according to a voltage of the DC source 76. Then, the luminance signal, having passed through the noise canceller 71, the YNR 72 and the picture-tone circuit 73, is mixed with a color signal in a mixing circuit 77 and becomes a video signal.
When the edit mode is used in such an arrangement, although a certain amount of noise remains, the high-frequency component of the luminance signal is not lost in the played-back image. This is because this arrangement, along with greatly limiting the effect of noise reduction, also makes the frequency characteristic of the luminance signal into a flat frequency characteristic. Consequently, when the dubbed image is played back, the remaining noise is eliminated by the picture quality correction of the play-back apparatus, and diffusion of the image is prevented, thus preventing deterioration in picture quality due to dubbing.
However, recent years have seen a large variation in quality of commercially available video-tapes. If recording and play-back are performed using poor quality video-tapes, the amplitude of the FM signal becomes small. As a result, the played-back image acquires a hard overall picture quality and noise begins to stand out. Of course, if recording and play-back are performed using good quality video-tapes, the played-back image acquires a soft overall picture quality and noise also does not stand out much. However, even with a good-quality video-tape, repeatedly performing recording and play-back causes the magnetic flux density of the tape surface to decrease, resulting in an increase in the tendency toward occurrence of noise in the image. For this reason, with the arrangement shown in FIG. 15, in order to obtain a good picture quality, it was hitherto necessary that the user correct picture quality by adjusting the gain of the picture-tone circuit 64 according to the quality of the video-tape. This had to be done by manipulating the picture quality adjuster VR.sub.61.
Moreover, with the arrangement shown in FIG. 15, even if picture quality is corrected with the picture quality adjuster VR.sub.61, the following problems occur, resulting in deterioration of image picture quality.
(1) If the frequency characteristic of the luminance signal is varied greatly by manipulating the picture quality adjuster VR.sub.61, the group delay characteristic is disturbed and phase shift of the image tends to occur. This causes problems like overshoot or smear.
(2) When the high-frequency component of the luminance signal is emphasized by manipulating the picture quality adjusters VR.sub.61, the noise component included in the high-frequency component also gets emphasized. Thus, although the played-back image has a high definition, noise stands out.
(3) When the high-frequency component of the luminance signal is attenuated by manipulating the picture quality adjuster VR.sub.61, the noise component included in the high-frequency component also gets suppressed. However, if the amount of noise suppression is excessive, although the noise no longer stands out in the played-back image, the definition of the image deteriorates and the S/N does not improve much.
On the other hand, with the arrangement shown in FIG. 19, since S/N is not very good in the currently available home VCRs, using the edit mode can in fact cause a deterioration of the S/N. Consequently, when a soft tape or the like, whose S/N has deteriorated due to repeated play-back, is dubbed using the edit mode, the deterioration of the S/N becomes so evident in the copied image that it is hard on the eyes. Moreover, although the edit mode is effective in that the group delay characteristic remains undisturbed due to the arrangement whereby the frequency characteristic is not corrected, there nevertheless exists a possibility of noise standing out when an image with an extremely bad S/N is used for dubbing. This is because noise suppression has been arranged to be limited.